During embryonic development, the atria will configure themselves in one of three ways: the atria may be normally attached to their respective ventricles e.g. the right atrium is attached to the right ventricle and the left atrium is attached to the left ventricle; the atria maybe transposed e.g. the right atrium is attached to the left ventricle and the left atrium is attached to the right ventricle, or the atria may be isomeric e.g. there are two left atria or two right atria, each attached to the ventricles e.g. a left atrium attached to a right ventricle and a left atrium attached to the left ventricle and vice versa (see heterotaxy). Recall that the right atrium has a large triangular shaped appendage, and the left atrium has a small tubular shaped appendage. Differentiation of the atria it is not easily done with echocardiography, and typically requires a TEE or MRI.
Under normal circumstances as the fetus develops, the pulmonary veins arise from the lung buds and will migrate towards and attach to a morphologic left atrium irregardless of whether the morphologic left atrium is attached to the right ventricle. the embryonic cardinal vein (a.k.a. the left vertical vein or the left caval vein) will eventually dissolve into other venous structures, leaving intact the superior and inferior vena cavae and the brachiocephalic (or innominate) vein, and should eventually divide into four pulmonary veins attached to a morphologic left atrium. If this does not occur, the primitive cardinal vein may result in a â€œconfluenceâ€ that drains venous flow into the right side of the heart. This is known as â€œtotal anomalous pulmonary venous returnâ€ and is a life-threatening cyanotic condition.
If the pulmonary veins attach normally to a morphologic left atrium but one or more of the pulmonary veins attach to the right atrium or the vena cava, this is known as â€œpartial anomalous pulmonary venous returnâ€.
Heterotaxy or atrial isomerism is a complex combination of congenital defects that range and severity and affect numerous organs and other physiologic systems. Concerning the heart, heterotaxy includes right atrial isomerism and left atrial isomerism. In other words, there are two right atria or two left atria.
A wide range of defects maybe present such as septal defects, valvular problems, asplenia, abnormalities of the liver, dextrocardia, conduction problems such as heart block and varying forms of anomalous venous return (refer to Total Anomalous Pulmonary Venous Return.
There are numerous physiologic problems associated with atrial dysmorphism, but for the purposes of this blog I would like to point out another defect, cor triatriatum. This is a subject for another blog, but this is something that is rare and I have run into on a couple of occasions.
Cor triatriatum is a membranous, partial, or complete blockage that can occur in either atrium, but most commonly in the left atrium. If it occurs in the left atrium, it is called â€œcor triatriatum sinisterâ€ and if it occurs in the right atrium, it is called â€œcor triatriatum dexterâ€
The best way to diagnose this abnormality is to use a bubble study
Thank you to my followers for explaining the difference between â€œsinisterâ€ and â€œdexterâ€.
Please refer to my book â€œCongenital Heart Defects, Simplifiedâ€ for more complete information